1. The Field of the Invention
The present invention relates generally to metal flake pigments. More specifically, the present invention relates to metal flake pigments having improved specular reflectance.
2. The Relevant Technology
Pigments are generally used to contribute to the optical and other properties of applications such as coatings, inks, extrusions, paints, finishes, glass, ceramics and cosmetics. Many varieties of pigments exist, some of which are metal flake based. These metal flakes comprise a thin film metal layer for improving the lustre, sparkle, shine, absorption, hiding and/or reflective properties of the application. The optical performance of the pigments, however, are duly constrained by the inherent limitations of each metal flake therein.
In general, it is known that for the application to achieve the greatest specular reflectance across visible wavelengths (300-800 nm), metal flakes should individually lay as flat as possible. As a collection of numerous flakes, the greatest reflectance, and hence greatest brightness, occurs when the flakes are collectively planar oriented to expose the greatest amount of surface area of the metallic flakes to the incident light and reflect as much of that light as possible.
A major factor, however, affecting those reflectance characteristics is the size or dimensions of the flake as the flake is used in a particular application. For example, if the flakes are thick, a plurality of thick flakes combined together in an application are prevented from lying together in a generally flat or horizontal singular plane because adjacent flakes cannot easily overlap each other due to their thickness. As a result, many flakes are adversely caused to be oriented in a substantially vertical manner and the plurality of flakes are formed into a radically non-planar layer. Incident light then exposed upon the non-planar layer is subject to extreme scatter and non-specular reflection. Thus, the favorable reflective properties of the application are diminished by thick flakes. To a lesser extent, thick flakes frequently cause other difficulties such as the clogging of automatic-spray paint guns during painting applications.
However, it is also well known that as the thicknesses of the flakes is reduced, the point is reached where the flakes become so flimsy (non-rigid, flaccid) that they begin to curl and/or wrinkle. This decreases favorable planarity and reflective properties because incident light exposed upon the flakes is subject to scatter and non-specular reflection. Additionally, if the flakes are too thin when applied onto a surface during applicational use, the flakes will assume any microscopic defects in the contour of that surface. For example, if that contour is rough, the flakes will correspondingly be rough or non-planar. Again, disfavored planarity and reflective properties result because incident light exposed on the surface is subject to scatter and non-specular reflection.
Some manufacturing processes form flakes from a singular, larger sheet or film of metal which is "fractured" by well known means into smaller, flake-sized particles. Two types of fracture may result, "ductile" or "brittle." Ductile fractures cause the metal to undergo substantial plastic deformation near the vicinity of fracture before fracture occurs. This deformation causes numerous malformed regions having disfavorable planar characteristics to appear. As before, these malformed regions, such as regions having curled or wrinkled metal, disadvantageously tend to scatter and diffuse incident light exposed thereupon. Brittle fractures, on the other hand, tend to cause little or no plastic deformation of the metal before the fracture occurs which enables the produced metal flake to maintain, as much as possible, the original planarity of the larger metal sheet. Consequently, it is desirable that brittle fracture occur during manufacturing.
However, brittle fracture does not occur with most metals having high reflectivity. In fact, brittle fracture is only likely to occur with materials having a large compressive strength as compared to its corresponding tensile strength. This is because the internal bond strength distributed throughout a material is composed of tensile and compressive components. The tensile strength compensates for forces out of the plane of the material and the compressive strength is related to forces in the plane. Thus, similar compressive and tensile strengths will allow ductile deformations since the relative strength into and out of the plane is equivalent. In contrast, brittle deformation occurs when the compressive strength is greater than the tensile strength and the material strength is directed into the plane, not out of the plane. Consequently, a high compressive strength relative to tensile strength results in bond rupture and material cracking when a force is applied. Thus, aluminum, for example, which has a tensile strength of about 13-24 lb/in.sup.2 and a compressive strength of about 13-24 lb/in.sup.2, would most likely undergo a ductile fracture under a uniaxial stress which would cause the aluminum to exhibit disfavored reflective characteristics. Moreover, once the aluminum is bent or deformed, as would occur with ductile fracture, the aluminum remains deformed and the disfavored reflective characteristics would persist. Consequently, it is difficult to manufacture metal flakes, such as aluminum, without malformations that reduce reflectance.
As is well known, fracture mechanics are not only important for metal flakes during the manufacturing process, but are as equally important during use. For example, applicational processes, such as the drying of a paint or ink solvent, also induce stresses on the flake. These stresses, caused by surface tension, again cause the flake to undergo fracture or malformation. Since brittle fracture of the flake during the applicational process also tends to produce smaller flakes that maintain much of the original planarity of the larger flake, instead of curled or deformed flakes, flake planarity and reflective properties are improved. Thus, flake brittleness is a characteristic not only preferred during the manufacturing process but also preferred during the applicational use. Accordingly, the prior art has attempted to produce thin, rigid and brittle flakes by facilitating both the manufacturing thereof and the reflective properties of the application.
Yet all prior solutions have involved compromises. For example, in U.S. Pat. No. 5,198,042, entitled "Aluminum Alloy Powders for Coating Materials and Materials Containing the Alloy Powders," it is taught to alloy the metal flake with other materials and metals to reduce the adverse curling, wrinkling and malleability of thin flakes. Alloying, however, dilutes the reflectance properties of the flake.
In U.S. Pat. No. 4,213,886, entitled "Treatment of Aluminum Flake to Improve Appearance of Coating Compositions," a surface bound species that pulls the flake flat in a coating resin is disclosed. This method, however, requires chemical tailoring of the flake and the resin in order achieve chemical compatibility with the species. Such compatibility is difficult and has not proved to be practical.
In U.S. Pat. No. 4,629,512, entitled "Leafing Aluminum Pigments of Improved Quality," flakes are floated on a resin coating. Adversely, this method submits the flake to durability attacks because the flake is unprotected. Such attacks primarily include corrosion which not only corrodes the flake but tends to give the application a mottled or discolored appearance. Additionally, if this method were used in conjunction with another resinous application, such as a clear overcoat paint, the overcoat itself would tend to disfavorably disrupt the planar orientation of the flake because of solvent penetration. Again, reflectance properties are decreased.
In U.S. Pat. No. 5,593,773, entitled "Metal Powder Pigment," pre-cracked flakes are disclosed having such a small aspect ratio that malformation of the flake is essentially impossible. A shrinking aspect ratio, however, also correspondingly shrinks the inherent reflectance capability of the flake. This is because, as the aspect ratio becomes smaller, any non-planar flake orientation during applicational use exposes such a small surface area of the flake to the incident light that reflection of that light is minimal. Other prior art proposals, such as encapsulating a metal flake in a metallic coating, also decrease the flake aspect ratio which adversely eliminates the ability of the flake to reflect incident light.
In U.S. Pat. No. 3,622,473, entitled "Method of Providing Aluminum Surfaces with Coatings," flake rigidity is increased by oxidizing the reflector of the flake to form a rigid, outer oxide layer. Whenever an oxide is used, however, the inherent reflectance properties of the flake are decreased. Additionally, oxides are typically formed at defect sites on the flakes which then tend to prevent a uniform application across the surface of the flake. This non-uniformity introduces a reduction in reflectance and can also cause a mottled applicational appearance.
Various attempts have been made to improve flake rigidity by applying singular or multiple layer coatings about the surfaces thereof Thus far, the singular layer coatings have been so thick that reflective properties are detrimentally damaged because the coatings have greatly contributed to the scatter of light. The multiple layer coatings have induced even more scatter and adversely caused light to diffuse at the boundaries between various layers. In addition, all coatings thus far have essentially been organic, and inherent within the crystalline structure of these organic coatings is a natural limitation as to how thinly applied the coatings can be manufactured and still provide structural rigidity to a flimsily thin metal flake. Disadvantageously, the natural thickness limitation is still so large that other applicational processes remain burdened by this thickness. Such processes include spraying the flakes through an automatic-spray paint gun. Moreover, organic coatings when applicationally used in a solvent are eventually caused to lose structural rigidity because of dissolution related effects.
Although some reflective coatings exist that are rigid and facilitate brittle fracture, the coatings are unlike most of the other prior art because they do not even use a metal flake. In U.S. Pat. No. 4,309,075, entitled "Multilayer Mirror with Maximum Reflectance," for example, multiple layer coatings are taught that merely simulate a metal flake and its reflective properties. These coatings, known as "high-low" coatings because of their alternating layers of high and low indices of refraction, are used to create a reflector that simulates the reflective properties of a metal flake. Another example is described in U.S. Pat. No. 3,123,490 issued to Bolomey, in which a layer of ZnS is coated on a top and bottom thereof with MgF.sub.2. Although rigid and subject to brittle fracture, this structure is typically very thick (about 215 nm) and cannot be used in many applications requiring thin flakes. Moreover, it is often necessary to have numerous layers of alternating high-low coatings to achieve simulation of the metal flake. But as thicknesses and layers increase, manufacturing complexities and economic burdens correspondingly increase.
Accordingly, it is desirous to find alternatives for inexpensively producing a thin, rigid and brittle metal flake having improved reflective characteristics thereby improving reflectance of metal flake-based pigments.